1. Field of the Invention
The present invention relates to an air-fuel control device for an engine which can be operated on an air-fuel mixture having either a rich or a lean air-fuel ratio.
2. Description of the Related Art
Engines operated on a lean air-fuel mixture having an air-fuel ratio higher than a stoichiometric ratio in the main operating range are known as lean burn engines. These lean burn engines are usually operated on a lean air-fuel mixture and are switched to operation on a rich air-fuel mixture when an acceleration or a high load operation is required.
Some lean burn engines are also equipped with swirl control valves, to thereby obtain a better combustion of the lean air-fuel mixture, and usually, these engines are provided with two inlet air passages for each engine cylinder; one leading to a helical inlet port of the cylinder, which generates a swirl of the inlet air therethrough in the cylinder, and the other leading to a conventional low pressure drop straight type inlet port.
The swirl control valve is provided in the inlet air passage of the straight port, for blocking the air passage in accordance with the load condition of the engine. For example, when the engine is operated at a low speed and low load, the swirl control valve is closed to block the inlet air passage to the straight port, and the amount of fuel injected and the ignition timing are adjusted to obtain a lean air-fuel mixture operation. When the air passage to the straight port is blocked, all of the inlet air to the engine flows into the engine cylinder through the swirl inlet port, and thus a strong swirl of an air-fuel mixture is generated within the cylinder, and therefore, a stable combustion can be obtained with a lean air-fuel mixture.
When the engine is operated at a high load, high speed condition, the swirl control valve is opened to allow inlet air into the cylinder through the low pressure drop straight port, and the amount of fuel injected and the ignition timing are adjusted to obtain a rich (or stoichiometric) air-fuel ratio mixture. Accordingly, the engine output is increased due to the increased inlet air flow and richer air-fuel ratio.
This type of the engine is disclosed, for example, by Japanese Unexamined Patent Publication No. 60-237140. In this engine, the switching of the swirl control valve and the air-fuel ratio is initiated by the degree of opening of the throttle valve; i.e., when the degree of opening of the throttle valve becomes larger than a predetermined value, the swirl control valve is opened and the air-fuel ratio is adjusted to obtain a rich mixture.
Typically, the operation of the swirl control valve and switching of air-fuel ratio are controlled by parameters representing an engine load (such as the degree of opening of the throttle valve) or engine speed, or both. FIG. 7 is a diagram indicating a typical operation mode of the engine. As shown in FIG. 7, the engine operation mode is switched to a rich mixture mode in which the swirl control valve is opened, and the amount of the fuel injected and the ignition timing are adjusted to a rich air-fuel mixture operation when the degree of opening of the throttle valve SV becomes larger than a predetermined value SV.sub.0, or the engine speed NE becomes higher than a predetermined value NE.sub.0. Due to the current demands for a low fuel consumption, it is desired to broaden the range of a lean air-fuel mixture, as shown in the typical example in FIG. 7, in which the predetermined value SV.sub.0 for the degree of opening of the throttle valve is set at 60-80%, and the predetermined value NE.sub.0 for the engine speed is set at about 4000 rpm.
In the lean mixture operation in which the swirl control valve is closed, however, the volume of the inlet air flow is much lower than in a rich mixture operation, because all of the inlet air flows into the cylinder through the swirl inlet port, and thus a higher pressure loss occurs than in the straight type port.
Also the engine output torque per unit inlet air volume is lowered due to the lean air-fuel ratio of the mixture, and accordingly, a torque generated by the engine becomes low during the lean mixture operation. Therefore, to obtain the same rate of acceleration, the driver must depress the accelerator pedal by a larger amount during the lean mixture operation than during a rich mixture operation. Namely, if an acceleration is required when the engine speed is close to NE.sub.0 (points A and B in FIG. 7), the amount of depression of the accelerator pedal is larger at point A (.DELTA.SV.sub.1 in FIG. 7) than at point B (.DELTA.SV.sub.2 in FIG. 7), even if the loads and the speeds of the engine before the start of the acceleration are almost the same at points A and B. This difference in the operation is confusing to the driver of the vehicle, and further, since an acceleration is frequently required in the middle engine speed range, i.e. close to NE.sub.0, the frequent occurrence of this difference in operation of the accelerator pedal causes the driver to become uneasy.
To solve this problem, it is possible to set the SV.sub.0 and NE.sub.0 lower values, so that the engine is always operated on the rich mixture in the operation range in which acceleration is frequently required. But this broadens the range of the rich mixture operation, and thus the fuel consumption of the engine is worsened because the engine is operated more frequently in the rich mixture mode.